Accurate Frequency Measurements Research Articles

Methanol and the optically pumped far-infrared laser

New results on the generation and spectroscopic analysis of optically pumped far-infrared (FIR) laser emission from CH/sub 3/OH have been obtained as part of a systematic study of methanol isotopomers as FIR laser sources utilizing the extended line coverage available from a recently developed high-resolution CO/sub 2/ laser of high efficiency. For normal CH/sub 3/OH, six new short-wavelength lines have been found using a 2 m long Fabry-Perot FIR laser cavity. Accurate heterodyne frequency measurements are reported for 14 CH/sub 3/OH FIR laser lines, nearly all above 100 cm/sup -1/, as well as accurate frequency offsets for most of the corresponding CO/sub 2/ pump lines. Spectroscopic assignments are presented for nine high-frequency FIR laser lines in four pump systems.

New far-infrared laser lines and assignments for ^13CH_3OH methanol

The 13CH3OH isotopomer of methanol has been reinvestigated as a source of far-infrared (FIR) laser emission by the use of the extended line coverage available from an efficient CO2 laser recently developed for optical pumping. With this system we have observed 19 new FIR laser lines, pumped by 11 different CO2 transitions, with five of the pump lines belonging to the new 9HP CO2 hot band. Accurate heterodyne frequency measurements of the FIR laser and the pump offset have been made for 16 FIR laser lines, including three previously reported in the literature. High-resolution Fourier-transform spectroscopic results for the CO-stretching and the CH3-rocking infrared fundamentals of 13CH3OH have been applied to assign the pump and lasing lines for five of the transition systems. One system has the novel feature that two different CO2 pump lines excite the same upper level, and hence two FIR laser lines of the identical frequency are observed for two different pump lines.

Technique for measuring grating frequency

Experimentally the real and imaginary parts of the Fourier spectrum are separated and used to detect the nature of the object, i.e., whether the object is an odd or even function or a combination of both. A modified Twyman-Green setup is used for this purpose. This type of splitting of the object function into even and odd parts may become useful in different fields of image processing. As an application the same technique is used for accurate measurement of the grating frequency.

CO-laser side-band spectrometer: Sub-Doppler heterodyne frequency measurements around 5 �m

We report the realization of a tunable sub-Doppler heterodyne spectrometer with high absolute accuracy, employing side-band generation with a CO laser. The fixed-frequency CO-gas laser, working from 4.7 to 8.4µm, is made partially tunable by the use of microwave side-band generation in a CdTe Electro-Optical Modulator (EOM). This leads to tunable radiation of high spectral purity. We describe the design of the microwave EOM, adapted to the CO laser, its performance and its first application to highly accurate frequency measurements. The side-band radiation is used for sub-Doppler stabilization of the CO laser, while the carrier frequency is mixed with the frequencies of two CO2 reference lasers. As a first result, we present measurements of OCS transitions in the 4.9µm (61 THz) region, reaching an absolute accuracy of 30 kHz (Δν/ν = 5×1O−10). Further application of our spectrometer to calibration gases will establish a variety of InfraRed (IR) calibration standards with a new quality of accuracy.

Frequency determination of far infrared optically pumped laser lines of vinyl fluoride using a niobium nitride Josephson mixer

Using the alternating current Josephson effect in a thin film niobium nitride (NbN) Josephson tunnel junction, we report the first accurate frequency measurements of optically pumped far infrared laser lines of vinyl fluoride (C2H3F). High harmonics of the local oscillator microwave source were mixed with the vinyl fluoride laser line in the junction and the intermediate frequency beat measured. The precise laser frequencies, and their CO2 pump lines are 902.001 593(42) GHz, 10P6; 889.341 248(67) GHz, 10P38; 673.997 567(19) GHz, 10R20; 634.221 681 (47) GHz, 10P20; 453.652 59(11) GHz, 10P24; and 446.698 886(30) GHz, 10P36. These frequencies are currently being used to analyse laser magnetic resonance spectra of the CF and CH2Br free radicals.

CH 318OH: FIR laser line frequency measurements and assignments

Observations of far-infrared (FIR) laser emission and assignments of FIR laser lines have been reported in the literature for the O-18 isotopomer of methanol optically pumped by CO 2 lasers. So far, wavelengths have been measured for 100 FIR laser lines, and assignments for 26 of these lines have been proposed. However, some assignments were uncertain, with several possible alternatives, notably for the lines pumped by the 10R(20) CO 2 line. The present work combines new accurate FIR laser heterodyne frequency measurements with the results of Fourier transform spectroscopic studies of the CO-stretching and FIR torsion-rotation bands of CH 3 18 OH in a detailed analysis of the FIR laser emission. Precise frequencies have been determined for 22 of the laser lines, and five new laser lines have been observed. The spectroscopic results have led to unambiguous assignments for 20 of these lines and their confirmation through transition combination loops. The assignments of 13 of the lines are given for the first time. With the new results, the enigmatic 10R(20) situation has been almost completely resolved.

A homomorphic method for measuring secondary amplitude modulation.

A homomorphic processing, software implemented method for measuring secondary amplitude modulation (AM) is described. The accuracy of measurement of this method was tested on a series of synthetically produced AM and vowel signals. The homomorphic method provided highly accurate measurement of modulation frequency and modulation index.

Frequency stabilization of FDM optical signals using an electronic counter

A new scheme is demonstrated for the frequency stabilization of FDM optical signals. This scheme uses an electronic counter to count signals detected by a scanning Fabry-Perot interferometer, instead of an A/D converter. The frequency measurement accuracy is within 50 MHz, and the frequency stability is within /spl plusmn/200 MHz over 50 hours. Using this scheme, it is possible to stabilize, with high accuracy, the frequencies of FDM carriers with unequal spacings. >

Adaptive algorithm for estimation of voltage phasor, frequency and rate of change of frequency

AbstractThis paper presents results in the design of an adaptive algorithm for real‐time estimation of voltage phasor, frequency and its rate of change. The new algorithm, separated in two stages, is based on the non‐recursive “Least Error Squares Technique” (LS Technique). The estimation of the unknown parameters of the voltage signal is improved by means of an update procedure in the first stage. The update procedure is iteratively provided by the algorithm adaptation to the actual estimated frequency so that the frequency measurement range and accuracy are significantly improved and sensibility to the noise is reduced. The rate of change of frequency, estimated in the second stage using a common LS Technique, is represented by the linearly time‐dependent observation model. To demonstrate the performance of the algorithm developed, computer simulated, experimentally obtained and real‐life data records tests are presented.

Application of Wigner-Ville distribution to measurements on transient signals

This paper deals with an application of the Wigner-Ville distribution (WVD) and with usual digital-processing techniques, such as the short-time Fourier transform (STFT), used in dedicated instrumentation for measuring nonstationary signals. The processed real signals are made analytic by means of Hilbert transformations; then suitable implementations of the windowed STFT and of the pseudo Wigner-Ville distribution (PWVD) in the time domain have been performed. Particularly, the fast Hartley transform (FHT) is used to evaluate the PWVD in the real domain. Furthermore, the use of an efficient interpolation algorithm and of a suitable flat-top windowing function is proposed in order to give accurate real-time frequency and amplitude measurements, respectively. With this aim, a dedicated digital system was set up, which uses the LabVIEW software to create virtual instruments (VI), suitable to process the data sequences. Finally, applications of the suggested techniques in analyzing noisy data were also investigated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characterization of pressure changes in the lower urinary tract during coughing with special reference to the demands on the pressure recording equipment

The exact demands on urodynamic equipment for measurement of coughs and cough associated pressure changes in the lower urinary tract have been analyzed from high-speed pressure recordings using a double microtip transducer and a storage oscilloscope. The equipment was tested in vitro by the step-test method. The natural frequency response was 175.6 Hz and the rise-time 2.5 ms, resulting in accurate measurements of frequencies up to about 60 Hz, which is way above the clinically measured frequencies. Four men and 2 women, all of whom were healthy volunteers, were examined in the supine position with an empty bladder. Pressures were measured in the bladder and in the external sphincter zone of the urethra. The spectral power density of the bladder and urethral pressures were calculated by Fourier analysis. The pressure changes in the urethra were in all volunteers equal to or slower than in the bladder. The analysis of the spectral power density showed that 99% of the pressure changes could be recorded with an instrument capable of recording 9 Hz frequencies, i.e., with a sampling rate of 18 Hz or more.

Atomic spectroscopy with diode lasers

Some applications of semiconductor diode lasers in atomic spectroscopy are discussed by describing different experiments performed with lasers emitting in the visible and in the near-infrared region. I illustrate the results obtained in the investigation of near-infrared transitions of atomic oxygen and of the visible intercombination line of strontium. I also describe how two offset-frequency-locked diode lasers can be used to excite velocity selective Raman transitions in Cs.I discuss the spectral resolution, the accuracy of frequency measurements, and the detection sensitivity achievable with diode lasers.

Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10−14.

Hyperfine structure and isotope shift in the far-infrared ground-state transitions of atomic oxygen.

We have investigated the $^{3}$${\mathit{P}}_{2}$${\ensuremath{\rightarrow}}^{3}$${\mathit{P}}_{1}$ and $^{3}$${\mathit{P}}_{1}$${\ensuremath{\rightarrow}}^{3}$${\mathit{P}}_{0}$ atomic fine-structure transitions in isotopically enriched samples of oxygen by means of tunable far-infrared spectroscopy. Accurate frequency measurements (a few parts in ${10}^{8}$) were made for all three stable isotopes and the $^{17}\mathrm{O}$ hyperfine structure was resolved. The experimental values for the isotopic shift of $^{18}\mathrm{O}$ relative to $^{16}\mathrm{O}$ for the J=2\ensuremath{\rightarrow}J=1 and J=1\ensuremath{\rightarrow}J=0 fine-structure transitions are in agreement with theoretical calculations.

A method for raising the accuracy of frequency measurement

A method is considered for making high-precision frequency measurements with simple, inexpensive, and inexact frequency meters.

Frequency control of semiconductor lasers

This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency. For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.

A detection technique for scanning force microscopy

A detection technique for use with noncontact attractive mode force microscopes is described. The technique relies on the accurate measurement of the resonant frequency of a cantilever probe as it rings in response to a very short excitation pulse. The resonant frequency reflects the spatially varying interactions between the probe and the specimen under study. This technique is for cantilevers with relatively high Q values (≳1000) as might be expected with vacuum operation. This technique has the advantage of being relatively easy to implement with commercially available instrumentation, while still providing excellent sensitivity and wide bandwidth operation. A sensitivity of 0.02 Hz at 75 kHz in a bandwidth of 50 Hz is comparable with other techniques. In addition, it is easy to accommodate a wide variety of cantilevers with different vibrational characteristics. A photomicrograph of the stray fields above bits written in a magnetic hard disk is presented, demonstrating the application of the technique to magnetic force microscopy.

Radio observations of the planets: The importance of laboratory measurements

In this paper we summarize the needs of planetary radio astronomers regarding future laboratory measurements. We present a short review of our present knowledge regarding planetary atmospheres and surfaces, and show our progression in data analysis over time when more and better laboratory data became available. To further constrain elemental abundances and understand the dynamics and cloud physics in the giant planet's atmospheres, we need laboratory data on the line broadening parameters of H2S gas under Uranian/Neptunian conditions, on the far wings of the H2S and NH3 line profiles, and on the dielectric properties of CH4−, NH3−, H2S−, and NH4SH‐ice. To obtain a better understanding of Venus' atmosphere, we need measurements of the absorption coefficient of gaseous H2SO4 at millimeter wavelengths. Accurate measurements of the center frequencies of the CO lines are necessary to determine wind velocity fields in Venus' and Mars' atmospheres. The modeling of radiative transfer through particulate material is the most difficult aspect of the analysis of microwave observations of planetary surfaces. Laboratory experiments that separately measure the absorption and scattering properties of lunar soils and/or terrestrial rock powders at frequencies from ∼1 to 200 GHz would provide a valuable addition to existing data at 450 MHz, 35 GHz, and far infrared frequencies. These data would be applied to the analysis of microwave spectra of planetary surfaces to help separate the effects of radiative transfer from those of nonlinear heat conduction and internal heat sources.

Far infrared laser lines and assignments of CH3OH: a review

Methyl alcohol is the most important lasing molecule in the Far-Infrared (FIR) spectral region, and the most widely used for investigation and for applications. Since the last critical review of 1984, over seventy papers have been published dealing with the FIR laser lines and the infrared spectroscopy of CH3OH. In 1984 we could list about 330 FIR laser lines, 98 of which were measured in frequency and 105 assigned. Since then more than 70 papers were published increasing the number of the known laser lines to 575 (103 measured in frequency). Also the FIR and IR spectroscopy was largely improved thanks to the analysis of high resolution FT spectra, and the number of the correctly assigned laser lines has been increased to 224. The wavenumbers of the assigned lines can now be predicted with an accuracy of about 0.001 cm−1 or better, thus approaching the accuracy of the experimental frequency measurements.

A vibratory sensor for locating parts

SUMMARYA vibratory sensor capable of locating parts by measuring their inertial properties is described in this paper. The sensor is designed to be fitted to the wrist of a robot and used to acquire parts from a stack or a tray. The initial coordinates of the parts need only be approximately known (say to ±25 mm and ±45°). The robot is thus said to operate in a “semi-ordered” environment which can be realised inexpensively as accurate pallets, fixtures or other parts presentation equipment are not required. Furthermore, the absence of complete disorder, as would exist with a “bin-picking” approach where parts are allowed to lie at random in a bin, reduces the degree of sophistication demanded of the sensor system. Consequently, the cost of the latter can also be kept low.The proposed sensor has a circular platform mounted on the shaft of a motor such that the shaft is normal to the platform. The part whose location is to be determined is held on the platform by magnetic or other means. The assembly consisting of the part, platform and motor is constrained to vibrate about an axis parallel to the platform. Two methods for computing the location of the part relative to the platform are described. Both methods require the natural frequencies of vibration of the part-platform-motor assembly for various angular positions of the platform. A simulation study to examine the effect of various design and operational parameters (inertias of the platform and part, stiffness of the spring restraining the movements of the vibrating assembly, location of part on platform, accuracy of frequency measurements) on the accuracy of the location computed is presented. The simulation results clearly demonstrate the feasibility of the sensor.